The present invention relates to an image fusing device for use in an image forming apparatus such as a copier, a printer and a facsimile machine, which fuses an unfused image formed on an image holding member such as a paper sheet and a film, and, in particular, relates to an image fusing device which thermally fuses an unfused image formed of a colored image forming material consisting mainly of thermoplastic resin on an image holding member.
Conventionally, as methods for fusing an unfused image of this type, there are typically known the following methods:
(1) Heating roll fusing method: There are provided a heating roll having a heater disposed at the center thereof and a pressure roll pressable against the heating roll. An image holding member carrying an unfused image is passed between these two rolls and the heat from the heating roll and the pressure between the two rolls are used to fuse the unfused image.
However, in the heating roll fusing method, because it is necessary to heat the heating roll having a large heat capacity, the heat energy consumption is large. Also, it takes time to heat the heating roll up to its operational temperature. Especially in a cold season, it takes a long time for the heating, disabling quick start. Another problem is that an image offset to the heating roll is likely to occur.
(2) Radiant fusing method: An unfused image containing a coloring agent is heated by use of radiant heat generated from a hot wire, to be fused thermally.
However, the radiant fusing method also has the disadvantage of large energy consumption, and there exists a problem on safety. For example, there is a possibility that an image holding member such as a paper sheet and a film catches fire.
(3) Solvent fusing method: An organic solvent vapor is sprayed in a gaseous state onto an unfused image formed on an image holding member and an image forming material is dissolved and fused.
However, in the solvent fusing method, the solvent gas may leaks out to cause environmental pollution or fire. That is, the solvent fusing method has a problem in the safety management.
(4) Pressure fusing method: A relatively high pressure is applied between two rolls, and an image holding member carrying an unfused image is passed between the two rolls to thereby fuse the unfused image onto the image holding member by pressure deformation.
However, in the pressure fusing method, image forming materials usable are limited to special ones. When the image holding member is folded, a fused image may be peeled off. That is, the pressure fusing method is disadvantageous in the stability of a fused image. Further, because the image holding member is deformed and deteriorated in the process of the pressure fusing, the double-sided fusing is difficult to realize. In addition, a fusing unit becomes heavy.
As described above, the several methods for fusing an unfused image have been proposed and implemented, but none of them are perfect. That is, each of them has its own advantages and disadvantages.
Recently, image forming apparatuses of various types have experienced improvements in some aspects: the image forming speed has been increased, its performance has been improved to produce clear pictures, and the size of the apparatus has been reduced. Thus, image fusing devices used in such image forming apparatuses have also been required to be compact, high in fusing speed, low in energy consumption, and free from the image offset problem. These requirements cannot be satisfied, in some cases, by conventional image fusing devices using any of the above image fusing methods, and therefore the development of a new type of image fusing device has been called for.